The present invention relates to methods and apparatus for detecting hidden peaks in wire fault location technologies and, more particularly, to methods for resolving overlapping and/or hidden peaks detected through spread spectrum time domain reflectometry (SSTDR).
Aircraft wiring problems have recently been identified as the likely cause of several tragic mishaps and hundreds of thousands of lost mission hours. Aircraft wiring is often routed behind panels or wrapped in special protective jackets and is not accessible, even during heavy maintenance when most of the panels are removed. A wire testing method that could test the wires continually, including while the plane is in flight would, therefore, have a tremendous advantage over conventional static test methods.
Various technologies in detecting and pin-pointing the wiring problems have been proposed and developed to address safety concerns, among which, spread spectrum time domain reflectometry (SSTDR) has received particular attention. SSTDR has demonstrated its potential as an effective way of locating intermittent faults on aircraft wires during flight.
In an advanced aircraft power distribution system, each section of the power bus and the feeder wires for every electric load is protected from the thermal (over current) stress by either a smart contactor or a remote power controller (RPC). Each of these over current options are equipped with certain level of intelligence to perform required functions, such as bus switching and load controls, bus and feeder wire over current protections, and arc fault detection (AFD). Therefore, in order to achieve comprehensive aircraft wiring integrity monitoring and fault location determination, the individual smart contactor or RPC becomes the perfect platform to incorporate an SSTDR sensor. FIG. 1 shows a conventional SSTDR sensor 100 having a transmitter 102 and a receiver 104 therewithin. The SSTDR sensor 100 may be connected to a power line 106 via a coupler 108.
The SSTDR technology for wire fault location determination follows the radar principle to identify the location of a fault. A modulated pulse signal is sent through a wire by the transmitter 102. The reflected signal due to a wire fault is then captured and decoded by the receiver 104. The distance from the wire fault location to the source of the original pulse signal is determined via timing of the return of the reflection relative to the original pulse and the speed of signal propagation inside the wire.
However, if under certain circumstances, the reflected signal overlaps with the original test signal, the determination of the timing of the return of the reflection relative to the original pulse becomes very difficult. The following two scenarios are described to illustrate these difficulties.
As shown in FIG. 2, when a fault 110 occurs at a location close to the range resolution, X, of the sensor 100, which is determined by the bandwidth of the sensor test signal, the reflected signal peak from the wire fault will be hidden in the transmitted loop back signal. In this case sensor might conclude a “no fault find”.
As shown in FIG. 3, in a practical aircraft power distribution system, a primary remote power controller 112 with a built-in SSTDR sensor 114 may be used to control a branched feeder network 116 to supply power to two electric loads 118, 120 controlled by two secondary (or downstream) RPCs 122, 124. If the distance between the RPC 112 and the RPC 122 is about the same as the distance between the RPC 112 and the location of the wire fault 126, the reflected signal peak due to the RPC 122 connection from branch A may overlap with that due to the wire fault 126 from branch B, leading to a false conclusion from the SSTDR sensor 114 that there is “no fault find”.
As can be seen, the range resolution of a SSTDR sensor depends on how closely a SSTDR sensor algorithm can resolve the two signal peaks when they are separated by small distance or overlapped with each other. If the hidden/overlapped peak issue is not properly resolved, a legitimate wire fault could be overlooked.
As can be seen, there is a need for a SSTDR wire fault method that is capable of resolving hidden/overlapped peaks.